1. Field of the Invention
The present invention relates to a display device using an organic light emitting element and a method of manufacturing the same. Specifically, the present invention relates to a display device and its manufacturing method in which an element substrate with an organic light emitting element formed thereon and a sealing substrate are placed close to each other with the sealing substrate opposing the element substrate in order to seal the organic light emitting element.
2. Description of the Related Art
Display devices using organic light emitting elements have been researched in recent years. A display device in which a pixel portion is composed of organic light emitting elements is self-luminous and, unlike liquid crystal display devices, does not need a light source such as back light. Therefore the use of organic light emitting elements is deemed as promising means to reduce weight and thickness of display devices and this type of display devices are expected to be used in cellular phones, personal portable information terminals (personal digital assistant: PDA), and the like.
Organic light emitting elements are luminous bodies which have a diode structure with an organic compound layer sandwiched between two electrodes and which emit light by combining electrons injected from one of the electrodes with holes injected from the other electrode in the organic compound layer. One of organic light emitting elements is an organic light emitting diode (OLED).
FIG. 12 is a sectional view showing a conventional active matrix display device that uses an organic light emitting element. Substrates 300 and 311 are transmissive of light. Of these substrates, the one that has an organic light emitting element is called an element substrate. An organic light emitting element 313 is composed of a pixel electrode 303, an organic compound layer 304, and an opposite electrode 305. The pixel electrode of the organic light emitting element is in contact with the top face of an interlayer insulating film 302, with inner walls of a contact hole that pierces the interlayer insulating film to reach a control circuit 301, and with the top of the control circuit. The control circuit 301 is composed of TFTs (thin film transistors), and has a switching TFT and a current controlling TFT. The switching TFT switches between conductive and unconductive in accordance with an output of a driving circuit. The current controlling TFT is applied a voltage according to an output of a driving circuit to the pixel electrode 303 so that a current flows between the opposite electrode and the pixel electrode. The intensity of light emitted from the organic compound layer 304 is dependent on the amount of current flowing between the pixel electrode and the opposite electrode.
A sealing substrate 312 is placed so as to face the element substrate, and is bonded using a seal member 306. When the pixel electrode is reflective of light whereas the opposite electrode is transmissive of light, emitted light travels upward in the sectional view and is visible through the light-transmissive sealing substrate. Since light emitted from the organic light emitting element can be taken out from the opposite side of the TFTs, display of high luminance and definition can be obtained irrespective of the aperture ratio of pixels.
If the display device is to display color images, a white light emitting diode is combined with a color filter. In this case, the sealing substrate has the color filter and the sealing substrate is bonded to the element substrate using the seal member. The sealing substrate is composed of the substrate 311, the color filter, and light-shielding portions 307. The color filter may be in contact with the white light emitting diode. The color filter is composed of a first spectroscope filter 308, a second spectroscope filter 309, and a third spectroscope filter 310. Each spectroscope filter transmits one of red light, blue light, and green light for a color display using three primary colors in an additive color mixing manner. The light-shielding portions 307 are provided in gaps between the spectroscope filters to block light. This structure is disclosed in Japanese Patent Application Laid-open No. 2000-173766, for example.
Development of display devices using organic light emitting elements is is advanced to obtain higher image quality by high luminance and color display as above. However, evaporation is used to form the organic compound layer and it is therefore difficult to deposit the layer at projections caused by wiring lines of the TFTs or the like and on side walls of the pixel electrode. If the continuity of the organic compound layer is broken at the projections or other places, short circuit of the pixel electrode and the opposite electrode is caused at that wiring breakage point to form a pixel that cannot emit light because no electric field is applied to its organic light emitting layer.
To counter this, a structure has been proposed in which a patterned insulating layer or an electric-insulating partition wall (hereinafter referred to as bank) is provided between pixel electrodes. In order to prevent dot defect due to a pixel that cannot emit light, a structure is proposed in which an insulating film is placed so as to cover the projections caused by the TFTs as well as the edges of the pixel electrode, and then the organic compound layer and the opposite electrode are formed on the top face of the insulating film and along the gently-sloped side faces of the insulating film. This structure is disclosed in, for example, Japanese Patent Application Laid-open No. Hei 9-134787. There is an overhanging bank with its top protruding further than its bottom. The structure that includes the overhanging bank is disclosed in Japanese Patent Application Laid-open Nos. Hei 8-315981 and Hei 9-102393, for example.
Still, there are problems remained unsolved in manufacturing a display device that uses an organic light emitting element.
For instance, the above-described structure of Japanese Patent Application Laid-open No. Hei 12-173766, where the sealing substrate is in contact with the organic light emitting element, is under fear of wiring breakage of the organic light emitting element when there is a hard foreign matter or a scratch on the sealing substrate. Then the opposite electrode and the pixel electrode short-circuit through the organic compound film to create a pixel that cannot emit light and lower the yield.
The foreign matter that often causes the wiring breakage is a hard one out of dust clung to the sealing substrate even after it is washed and dust mixed in during manufacture of the color filter. In some cases, dust in the interior of the apparatus or in the surroundings settles on the organic light emitting element in the manufacturing process of the element substrate.
The wiring breakage often takes place when the seal member is cured between the element substrate and the sealing substrate to bond the two substrates. A large pressure vertical to the substrate planes is applied when bonding the substrates. Therefore, if there is hard dust on the organic light emitting element, a large force is locally applied to cause the wiring breakage of the organic light emitting element. Furthermore, the organic light emitting element is damaged by shear stress when a pressure parallel to the substrate planes is applied during bonding and there is a hard projecting dust. Accordingly, an idea for avoiding the wiring breakage due to dust is to allow a gap between the organic light emitting element and the sealing substrate so that the element does not touch the sealing substrate.
However, when the gap between the organic light emitting element and the sealing substrate is wide, it makes it easy for moisture and oxygen to enter the display device from the sides thereof. The front and back of the display device, which are substrates formed of glass of inorganic materials or a metal, allow little steam and oxygen to transmit and therefore almost no moisture and oxygen enter the display device from its front and back. On the other hand, the seal member formed of an organic resin is provided on the sides of the display device between the substrates. Since the seal member has high moisture permeability, the amount of moisture that enters the sealed space through the seal member is too large to ignore. Accordingly, it is better if the height of the display device in section, excluding the thicknesses of the substrates, is smaller.
Smaller height is preferred because moisture oxidizes the cathode of the organic light emitting element or causes peeling of the organic compound layer off the cathode to generate dark spots (pixels that cannot emit light) and lower the display quality considerably. A dark spot is a progressive defect and it is said that it progresses even when the organic light emitting element is not in operation. This is because the cathode is formed of AlLi, MgAg, or the like, and alkaline metal or alkaline-earth metal contained in the cathode is highly reactive to moisture.
However, if the sealing substrate is placed above the organic light emitting element with a gap, there is no means to control the distance between the organic light emitting element and the sealing substrate to make the distance nonuniform throughout the display region. In the display device that takes out light emitted from the organic light emitting element from the sealing substrate side, the nonuniform distance between the substrates causes interference fringes to lower the visibility.
To summarize, when the contact between the organic light emitting element and the sealing substrate is coupled with the presence of a hard projecting foreign matter, wiring breakage of the organic light emitting element takes place to lower the yield. When the organic light emitting element is spaced apart from the sealing substrate, on the other hand, there is a problem of an increased amount of moisture entering the display device from the sides thereof, which accelerates degradation of the organic light emitting element. The organic light emitting element spaced apart from the sealing substrate can cause another problem of uneven brightness due to interference in the structure where light emitted from the organic light emitting element is taken out from the sealing substrate side.